Lithium niobate transducers

ABSTRACT

The specification describes ultrasonic transducers made from single crystal lithium niobate. Five crystal orientations are given for which transducer characteristics are especially favorable.

734-632 5? K /waa zaz gi 51 7A 'XR 315919513 United States 1 13591313[72] Inventors Gerald A.Coquin [56] References Cited Berkeley "Eights; Mh UNITED STATES PATENTS i": 3,423,686 1/1969 Ballman eta1.. 252/629Warner, Jr., Wluppany, all of, NJ.

I 3,348,077 10/1967 Nitsche 310/85 80338 2 490 216 12/1949 Jaff 310 9 s22 Filed Feb. 28 1969 e I 3,461,408 8/1969 Onoe et al. 310/95 X [451Paemed 2 486 187 10/1949 Masch 310/9 5 [73] Assignee Bell TelephoneLaboratories, incorporated Murray Hill, Berkeley Heights, NJ. PrimaryExaminer-Milton O. Hirshfield Assistant Examiner B. A. ReynoldsAtt0rneysR. J1 Guenther and Arthur J. Torsiglieri [54] LITHIUM NIOBATETRANSDUCERS v 7 Claims, 7 Drawing Figs.

[52] U.S.C| 310/95,

252/629 ABSTRACT: The specification describes ultrasonic transdu- [51]1nt.Cl HOlv 7/00 cers made from single crystal lithium niobate. Fivecrystal [50] Field of Search 3 l 0/9.5; orientations are given for whichtransducer characteristics are 252/629 especially favorable.

, PATENTEUJUL 6197i 3,591,813

SHEET 1.--UF 2 FIG.

G. A. COOU/N lNl/ENTORS AH. MEITZLER AW WARNER JR.

A TTORNFV PATENTED JUL 6 I87! SHEET 2 [IF 2 FIG. 4

LITHIUM NIOBATE TRANSDUCERS This invention relates to piezoelectriccrystals of lithium niobate and to ultrasonic transducers employingthese crystals.

Recent interest in crystals in the class (3m) has uncovered newpiezoelectric materials. Much interest has centered around lithiumtantalate in this regard (see Journal of the American Ceramic Society,48, 1 12 (1965)).

It has now been found that certain crystal orientations exist in lithiumniobate which for transducer applications have piezoelectric propertiessuperior to those previously obtained with lithium tantalate. Crystalsin class (3m) have a low degree of symmetry and the useful orientationshave not previously been recognized for LiNbo Useful orientations arethose that result in transducers vibrating in predominantly a singlemode and having a high effective electromechanical coupling factor. Afurther important property, which is favorably exhibited by lithiumniobate, is a low dielectric constant for all orientations.

Several crystal orientations have been found for LiNbQ, which resultintransducers having coupling factors in excess of 50 percent and whichhave sufficient modal purity that propagation of energy in spuriousmodes is more than 40 db. below the signal level of the main mode. Inaddition to the high level of performance obtainable from LiNbOtransducers, this material has the additional advantages that it ispresently available commercially and possesses mechanical propertiesthat enable it to withstand the processing required to prepare platessufficiently thin for high frequency applications.

The devices for which LiNbO is particularly suited are ultrasonictransducers operating at relatively high frequencies, i.e. above 100mHz. Ultrasonic delay lines for digital storage at those frequencies arecurrently being developed. High frequency ultrasonic transducers arealso useful in acoustooptic devices such as ultrasonic light deflectorsand ultrasonic light modulators.

The crystal orientations which form the basis for this invention arerotated Y-cut crystals having orientations designated (zxl 73, (yzw) l7,and (zxl) 54 and X-cut crystals designated (xyt) 41, and (xyt) -49.

These five crystal designs and their application will be described morecompletely in the following detailed description.

In the drawing:

FIGS. 1 to 5 are geometric representations of the inventive crystalorientations;

FIG. 6 is a perspective view of an ultrasonic transducer incorporatingone of the crystals of the invention; and

FIG 7 is a perspective view of an ultrasonic device incorporating thetransducer of FIG. 6.

Rotated Y-cut plates of LiNbO can vibrate in a pure shear mode withparticle displacement along the X-axis or in either a quasi-shear or aquasi-longitudinal mode of vibration, with particle displacement normalto the X-axis but neither normal nor parallel to the plane of the plate.An electric field applied in the thickness direction will not excite thepure shear mode but in most cases will excite both the quasi-shear andquasilongitudinal modes simultaneously. However, there are four rotatedY-cuts where an electric field in the thickness direction excites onlyone mode, the angles of rotation being 36, 90 (Z-cut), 123, and 163".Two of these four, the 36 and 163 cuts, are most useful for transducerapplications.

The 163 rotated Y-cut plate, a (zxl) +73 cut in IRE notation, has nocoupling to the quasi-longitudinal mode and an effective coupling factorof 61 percent for the quasi-shear mode. The particle displacement forthis mode is l.7 from the plane of the plate so that it is very near tobeing a pure shear mode of vibration. The fact that it is not exactly apure mode implies that, when used as a transducer, this cut will excitea small amount of longitudinal wave motion in addition to the main shearwave. If the longitudinal wave amplitude were large this could beobjectionable in some applications. However, since the angle of theparticle displacement is only 1.7 from the plane of the plate, thelongitudinal mode excitation is in this case negligible for practicalpurposes. For known transducer applications there are two basicallydifferent useful orientations: one with the length axis perpendicular tothe particle displacement vector, a (zxl) 73 cut; and one with thelength axis along the direction of the particle displacement vector, a(zyw) -l7l cut. The (zxl) 73 orientation is shown in FIG. 1. The (yzw)-1 7 orientation is shown in FIG. 2.

The 36 rotated Y-cut plate, a (zxl) -54 cut in IRE notation, is shown inFIG. 3. This crystal has zero coupling to the quasi-shear mode and aneffective coupling factor of 49 percent for the quasilongitudinal mode.The particle displacement for this mode is 3.8from the plate normal, sothat it is not quite as pure a mode of vibration as quasi-shear mode inthe 163 rotated Y-cut. This implies that such a transducer would excitea small amount of shear wave motion in addition to the main longitudinalwave. However, experiments conducted by bonding transducers of this typeto fused silica indicated that the shear wave signal was not observableand its amplitude was at least 40 db. down from the longitudinal modesignal.

LiNbO plate with faces normal to the X-axis, as defined by the 1949 IREstandard, can vibrate in a pure longitudinal mode with the particledisplacement along the X-axis, or in either of two pure shear modes withthe particle displacement normal to the X-axis. However, an electricfield applied in the thickness direction excites only the two shearmodes. F urthermore, one of the shear modes, which will be called thestrong shear mode, is excited much more efficiently than the other(weak) shear mode. The effective coupling constant is 68 percent for thestrong shear mode and only 10 percent for the weak shear mode. Thus anX-cut LiNbO transducer bonded to an isotropic delay medium would exciteone shear wave very strongly and the other shear wave would be about 18db. down. Since both shear waves in the isotropic material have the samevelocity, the net result is a very slight elliptical polarization of theparticle displacement, which is not objectionable.

The particle displacements of the two shear modes in the LiNbO are notalong the Y and Z crystal axes but are inclined to the crystal axes, thedirection of displacement for the strong shear mode being 41 from theZ-axis. r

There are two major types of ultrasonic delay lines using shear modetransducers and both of these types of delay lines require controllingthe direction of the particle displacement vector. Polygon delay linesrequire that the particle displacement vector lie in the planes ofincidence and reflection for a transverse wave reflecting from a facet.This, in turn, requires that the displacement vector be normal to thelong direction of a rectangular plate. Hence in the IRE notation, therequired plate is an (xyt) 41 cut. This crystal is shown in FIG. 4. Theother type of line is the strip or plate delay line in which thetransducers are again rectangular plates with the length usually five ormore times the height. For this type of delay line, the particledisplacement vector must be parallel to the major surfaces of the delaymedium; consequently, along the length direction of the transducer.Again according to the IRE notation, the required plate is an (xyt) 49cut. This plate is shown in FIG. 5.

In FIG. 6, a lithium niobate crystal, oriented as in one of FIGS. I to5, is provided with electrodes 2 and 3, which may be deposited, plated,etc. in accordance with any suitable technique. Such electrodes maycover the broad crystal faces as shown or may be of lesser area tominimize unwanted coupling. Electrical connection to the electrodes ismade by means of leads 4 and 5. The transducer of this figure may serveas a resonator, for example in performing the function of a filter orfrequency standard, or it may be part of a larger device such as a delayline.

The device of FIG. 7 is a conventional delay line incorporating atransducer 10 which, like the device of FIG. 6, is made up of a plate 11of LiNbO together with its associated electrodes 12 and 13. Electricalconnection is made by means of leads l4 and 15 connected to a signalsource not shown. The

- elastic wave produced by the electrical signal is then launched in theacoustic mediumlfi, which may be made of silica, glass, metal, or anyother suitable material. For certain uses, it is desirable to use LiNbOfor this member also, it having been observed that this material showsunusually low loss particularly for frequencies above 100 mHz. Uponreaching the end of acoustic member 16, the elastic wave is reconvertedinto an electromagnetic signal in transducer plate 21, and this signalis detected by means of circuitry including electrodes 22 and 23,together with wire leads 24 and 25. p

It should be stressed that the angle of rotation specified is criticaland should not be varied by more than :3". Deviations greater than thisresult in deleterious effects on the resonator characteristics such as areduction in the coupling efficiency and an increase in unwantedresonances.

The foregoing orientations have been consistently described as appliedto a plate structure, and it is this configuration that is of concern inmost transducers. For these purposes, a plate is generally about a halfwavelength thick for the center frequency, that is of the order ofmillimeters or less. The large dimensions are generally determined onthe basis of good device design, such as desired and/or permittedelectrode resistance, capacitance, etc. It is reasonable to assume thattransducer plates have large dimensions, at least five times thethickness dimension.

The invention has been described very briefly in terms of a small numberof embodiments. The composition itself and acceptable techniques forpreparing the composition are sufficiently well known so that detaileddiscussion is unnecessary. Fundamentally, the invention depends upon thefinding that the particular orientation described results in anoptimization of the properties disclosed. Minor modifications made inthe composition, due either to accidental inclusions or responsive to adesire to alter properties such as temperature depen dence,conductivity, absorption, growth, etc. do not alter the inventivefinding. Accordingly, the preferred orientation is considered to applyso long as at least 99 percent by weight of the composition is LiNbOSimilarly, representation of the vast family of suitable transducerstructures by the small number of examples set forth is not intended tolimit the invention.

While the devices described have utilized crystal sections faces awayfrom the plateau or, in the extreme, by use of one or two convexsurfaces. Still another approach, sometimes referred to as modetrapping, utilizes thickened electrodes afcrystal lying betweenelectrodes. Beveling ofa resonator plate to reduce unwanted resonancesis also practice. Accordingly, to benefit from the inventive teaching itis necessary only that those surface portions of the major facesassociated with the motion be oriented as specified, The annexed claimsare to be so construed.

Various additional modifications and extensions of this invention willbecome apparent to those skilled in the art. All such variations anddeviations which basically rely on the teaching through which thisinvention has advanced the art are properly consideredwithin the spiritand scope of this invention.

What we claim is:

l. A single crystal plate of lithium niobate having a crystalorientation selected from the following: (yzw) l7 (i3), (at!) 54 (i-3),(xyt) 41 (:32), (xyt) 49 (:3").

2. The single crystal plate 0 claim 1 having a (yzw) -l7 (:3")orientation.

3. The single crystal plate of claim 1 having a (zxl) 54 (:3orientation.

4. The single crystal plate of claim 1 havinga (xyt) 41 (:3 orientation.

5, The single crystal plate of claim 1 having a (xyl) 49 (:3orientation.

6. A crystal in accordance with claim 1 in combination with means forimpressing an electric field across the thickness of the crystal plate.

' 7. An ultrasonic device comprising a crystal in accordance with claim1, means for impressing an electric field across the thickness, and anultrasonic wave transmission medium in association with the crystal sothat elastic waves generated by said crystal propagate through thetransmission medium.

2. The single crystal plate of claim 1 having a (yzw) -17* ( + or - 3*)orientation.
 3. The single crystal plate of claim 1 having a (zxl) -54*( + or - 3*) orientation.
 4. The single crystal plate of claim 1 havinga (xyt) 41* ( + or - 3*) orientation.
 5. The single crystal plate ofclaim 1 having a (xyt) -49* ( + or - 3*) orientation.
 6. A crystal inaccordance with claim 1 in combination with means for impressing anelectric field across the thickness of the crystal plate.
 7. Anultrasonic device comprising a crystal in accordance with claim 1, meansfor impressing an electric field across the thickness, and an ultrasonicwave transmission medium in association with the crystal so that elasticwaves generated by said crystal propagate through the transmissionmedium.